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Keller settles for stone columns

A new combined CFA and stone column foundation has been developed to cope with soft ground and hard spots in east London.

KELLER GROUND Engineering has used a new foundation system, which combines CFA piles topped with vibro stone columns, for the first time on a housing project in Limehouse Basin in London Docklands.

Running along the north side of the basin, the cut and cover Limehouse link road tunnel was built in the early 1990s by top down construction, with compacted sand and gravel fill placed above it. The main challenge now is to build the houses over the tunnel, which is 4m below ground level, with minimal impact on the tunnel.

Houses are being constructed on four 28m wide, 750mm thick foundation slabs separated by 5m gaps and designed as a cantilever structure to reduce the loading on the tunnel.

The problem was a soft layer of silty alluvial clay predicted to cause settlement problems. This was underlying the compacted (and some older, uncompacted) fill beneath a narrow strip of land between the north side of the tunnel and the Docklands Light Railway, which runs on viaduct along the site's northern boundary.

Piling from ground level was rejected as it would have created too rigid a structure, so Keller came up with a combin- ation of CFA piles topped with stone columns.

The stone columns cause an arching effect in the fill, transferring the load onto the piles, which bridge across the soft material and support the full load of the housing and the fill, significantly reducing settlement from the predicted 100mm to around 20mm and reducing the rigidity of the substructure.

Over 400, 450mm diameter CFA piles, with design loads of 500kN, were installed on a 1.5m triangular grid over a five week contract in July and August this year. These pass through the fill and the soft layer down to 17m, founding in stiff clays of the Lambeth Group. The piles were cut off at between 1.5m and 2m above the top of the soft layer.

After each pile was installed, a bottom feed vibro rig followed on - after a four hour wait - to place the stone column, which keys 0.5m into the top of the pile. A bottom feed machine was necessary because it allowed installation below the high water table.

Surface works are now well under way. Main contractor Wates Construction, working for client Belway Homes has started two of the three, seven storey blocks of luxury apartments overlooking the basin.

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